Re: [PATCH 6.12.y] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo <harry.yoo@oracle.com>]

Please don't apply this, I made a mistake while backporting.
(Thanks to Pedro for catching it!)

I'll resend the backport for v6.12, v6.6, v6.1, and v5.15 tomorrow.

Leaving an inline comment below on what I got wrong below...

On Mon, Sep 08, 2025 at 10:09:31AM +0900, Harry Yoo wrote:
> Introduce and use {pgd,p4d}_populate_kernel() in core MM code when
> populating PGD and P4D entries for the kernel address space.  These
> helpers ensure proper synchronization of page tables when updating the
> kernel portion of top-level page tables.
> 
> Until now, the kernel has relied on each architecture to handle
> synchronization of top-level page tables in an ad-hoc manner.  For
> example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for direct
> mapping and vmemmap mapping changes").
> 
> However, this approach has proven fragile for following reasons:
> 
>   1) It is easy to forget to perform the necessary page table
>      synchronization when introducing new changes.
>      For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory
>      savings for compound devmaps") overlooked the need to synchronize
>      page tables for the vmemmap area.
> 
>   2) It is also easy to overlook that the vmemmap and direct mapping areas
>      must not be accessed before explicit page table synchronization.
>      For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated
>      sub-pmd ranges")) caused crashes by accessing the vmemmap area
>      before calling sync_global_pgds().
> 
> To address this, as suggested by Dave Hansen, introduce _kernel() variants
> of the page table population helpers, which invoke architecture-specific
> hooks to properly synchronize page tables.  These are introduced in a new
> header file, include/linux/pgalloc.h, so they can be called from common
> code.
> 
> They reuse existing infrastructure for vmalloc and ioremap.
> Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK,
> and the actual synchronization is performed by
> arch_sync_kernel_mappings().
> 
> This change currently targets only x86_64, so only PGD and P4D level
> helpers are introduced.  Currently, these helpers are no-ops since no
> architecture sets PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> 
> In theory, PUD and PMD level helpers can be added later if needed by other
> architectures.  For now, 32-bit architectures (x86-32 and arm) only handle
> PGTBL_PMD_MODIFIED, so p*d_populate_kernel() will never affect them unless
> we introduce a PMD level helper.
> 
> [harry.yoo@oracle.com: fix KASAN build error due to p*d_populate_kernel()]
>   Link: https://lkml.kernel.org/r/20250822020727.202749-1-harry.yoo@oracle.com
> Link: https://lkml.kernel.org/r/20250818020206.4517-3-harry.yoo@oracle.com
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Acked-by: Kiryl Shutsemau <kas@kernel.org>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: bibo mao <maobibo@loongson.cn>
> Cc: Borislav Betkov <bp@alien8.de>
> Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
> Cc: Dennis Zhou <dennis@kernel.org>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Dmitriy Vyukov <dvyukov@google.com>
> Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jane Chu <jane.chu@oracle.com>
> Cc: Joao Martins <joao.m.martins@oracle.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Cc: Liam Howlett <liam.howlett@oracle.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Thomas Gleinxer <tglx@linutronix.de>
> Cc: Thomas Huth <thuth@redhat.com>
> Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>  include/linux/pgalloc.h | 29 +++++++++++++++++++++++++++++
>  include/linux/pgtable.h | 26 ++++++++++++++++++++++----
>  mm/kasan/init.c         | 12 ++++++------
>  mm/percpu.c             |  6 +++---
>  mm/sparse-vmemmap.c     |  6 +++---
>  5 files changed, 63 insertions(+), 16 deletions(-)
>  create mode 100644 include/linux/pgalloc.h
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 1ba6e32909f8..e7d5c02ac0fb 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1343,6 +1343,23 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
>  	__ptep_modify_prot_commit(vma, addr, ptep, pte);
>  }
>  #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> +
> +/*
> + * Architectures can set this mask to a combination of PGTBL_P?D_MODIFIED values
> + * and let generic vmalloc, ioremap and page table update code know when
> + * arch_sync_kernel_mappings() needs to be called.
> + */
> +#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> +#define ARCH_PAGE_TABLE_SYNC_MASK 0
> +#endif
> +
> +/*
> + * There is no default implementation for arch_sync_kernel_mappings(). It is
> + * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> + * is 0.
> + */
> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> +
>  #endif /* CONFIG_MMU */

This is already present in include/linux/pgtable.h (due to "mm: move page
table sync declarations to linux/pgtable.h"), and my backport mistakenly
duplicated this hunk.

-- 
Cheers,
Harry / Hyeonggon